1. Field of the Invention
This invention relates to an automatic sheet feeding device for properly feeding sheets different in size and thickness one by one to various types of printing apparatuses such as a general purpose printer, a facsimile machine, and a copying machine, and more particularly, to an automatic sheet feeding device having a single sheet feeding stacker capable of selectively stacking either of two kinds of sheets of different type in size and thickness, i.e. an ordinary typing or copying paper being relatively thin and a postcard being relatively thick, and automatically feeding the sheets thus stacked to the printing apparatus one by one.
2. Description of the Prior Art
There are a variety of sheet feeding devices for automatically feeding sheets of plastic, paper or the like one by one to a printing apparatus such as a general purpose printer for printing data outputted from a wordprocessor, computer, etc. and facsimile and a copying machine. Typical of these sheet feeding devices capable of stacking stacks of sheets different in size is a sheet feeding stacker having two or more paper bins and adapted to feed the sheets stacked thereon to a printing portion of the printer one by one, and a sheet ejecting stacker for holding printed sheets discharged from the printer. In order to feed the sheets stacked in the sheet stacker to the printer one by one, there has been generally adopted a mechanism composed of one or more sheet feed rollers which are in contact with the uppermost one of the stacked sheets in the sheet feeding stacker and a pair of sheet separation claws striking against the forward top corners of the stacked sheets. Upon reception of a sheet feeding instruction from the printer, the sheet feeding device is operated to rotate the sheet feed rollers so as to send out the uppermost one of the stacked sheets toward the printer. At this time, though the forward corners of the uppermost sheet are pressed with the separation claws, the uppermost sheet is flexibly bent while being retained by the separation claws, and then sprung to be released from the separation claws by rotating the sheet feed rollers in the sheet feeding direction. As a result, only one sheet thus released can be sent out toward the printing portion of the printer. Such a mechanism for feeding sheets one by one by use of the sheet separation claws can be applied to a relatively thin sheet such as ordinary copying or typing paper, but is awkward in dealing with a relatively thick, stiff sheet such as a postcard and cardboard. If the sheet feed rollers, which are in touch with the uppermost of such stiff sheets stacked on the sheet feeding stacker and held by the separation claws as noted above, is rotated in the sheet feeding direction in order to thrust the aforesaid uppermost sheet forward, the uppermost sheet cannot be flexibly released from the sheet separation claws because of its stiffness. When the sheet is forcibly thrust forward by rotating the sheet feed rollers in the sheet feeding direction, it will suffer a disadvantage in that the forward corner portions of the uppermost sheet are folded or damaged. Therefore, so far there have been properly used a mechanism having sheet separation claws as noted above for feeding a relatively thin, flexible sheet and a mechanism having a sheet through gap means for permitting one relative thick, stiff sheet to pass therethrough. That is to say, a conventional sheet feeding system as disclosed in Japanese Patent Application Public Disclosure SHO 63(1988)-60830(A), for example, adpts an auxiliary sheet cassette capable of containing a stack of thick sheets. This sheet cassette is provided with a sheet separation mechanism composed of a sheet pass through gap means as discussed above and can be detachably fitted in a standard sheet stacker integrally formed in the sheet feeding system. Otherwise, as seen in a conventional copying machine, there have been selectively used a standard sheet cassette for containing relatively thin sheets such as a copying paper and another sheet cassette for containing relatively thick sheets such as a postcard. That is, a sheet separation means for each cassette is different. Thus, the conventional sheet feeding device necessitates at least two sheet cassettes or stackers for stacking different sorts of sheets and calls for the troblesome work of exchanging the sheet cassettes particularly when being applied to the sheet different in thickness. Though there has hitherto been an idea of combining a sheet stacker for a thin sheet with a sheet stacker for a thick sheet, a mechanism having both the sheet stackers becomes complicated in structure and cannot be expected to be stably operated.